Reversible phase change compositions of calcium chloride hexahydrate with potassium chloride

ABSTRACT

A reversible liquid/solid phase change composition comprising a mixture of hydrated CaCl 2  and KCl in which the KCl modifies the semi-congruently melting behavior of CaCl 2 .6H 2  O to the extent that the mixture approaches congruent melting behavior. The composition preferably includes nucleating additives to modify and suppress the supercooling properties of the liquid phase of the composition. The composition most preferably comprises an admixture of hydrated CaCl 2  and KCl having an amount of NaCl and/or SrCl 2 .6H 2  O as an additive sufficient to obtain an effectively congruently melting mixture.

BACKGROUND OF THE INVENTION

Phase change materials (PCM's) in which the heat of fusion of various hydrated salt compositions is employed are well known in the literature. In the ASHRAE Journal of September, 1974, entitled SOLAR ENERGY STORAGE, Dr. M. Telkes evaluated the thermal, physical and other pertinent properties of PCM's on the basis of economics, applicability, corrosion, toxicity and availability for large scale installations. Among the materials evaluated were various salt hydrates and their eutectics including CaCl₂.6H₂ O which undergoes several phase transitions to materials of different crystal structure, i.e. CaCl₂.6H₂ O to CaCl₂.4H₂ O+2H₂ O at 29° C.

When heated to a temperature of above 33° C., the salt CaCl₂.6H₂ O dissolves completely in its water of crystallization. When cooled, formation of four different crystal forms is possible, i.e., CaCl₂.6H₂ O and three forms of CaCl₂.4H₂ O. If any of the 4H₂ O crystals form, the heat of fusion is much less than 46 cal/gm (CaCl₂.6H₂ O in substantially pure form undergoes a liquid/solid phase transition at about 30° C. releasing or alternately absorbing about 46 calories of heat per gram). Despite the relatively low cost of CaCl₂, the formation of its four different crystal forms was deemed to be disadvantageous.

Carlsson et al., in Swedish Pat. No. 410,004, claim a method for suppressing the tetrahydrate formation during repeated melting and crystallization of a system based on CaCl₂.6H₂ O. In a comparative study, Carlsson et al., determined that in solutions in the concentration range of from 48 to 53 weight percent CaCl₂, using CaCl₂.6H₂ O of highest purity, the crystallization temperatures for CaCl₂.6H₂ O and CaCl₂.4H₂ O where such that the solution was incongruently melting and that CaCl₂.4H₂ O crystallized and precipitated out of the solution thus losing its heat storage capacity. By using a solution of the same concentration from CaCl₂ of technical grade (Road Salt) containing NaCl and KCl as impurities, the solubility of the tetrahydrate decreased and that of the hexahydrate increased and on repeated melting and crystallization, the precipitation becomes significant and the system again loses its heat storage capacity. Thus, the conclusion can be drawn that the use of technical grade CaCl₂ (Road Salt) results in a poorer performance due to a relative increase in tetrahydrate formation as compared to a system based on high purity CaCl₂. Carlsson et al., discovered that the addition of one or more compounds, including about 2 weight percent SrCl₂.6H₂ O, increased the solubility of the tetrahydrate and suppressed tetrahydrate formation on repeated melting and crystallization. The amount of addition was found to be dependent upon the amount of impurities present in the system, which in an example using Road Salt was determined to be 2.2 weight percent.

The relative amounts of each impurity in the technical grade salt (Road Salt) was not determined nor was it held to be important to the outcome of the tests conducted. In fact, the use of Road Salt was found to be less desirable from the standpoint of tetrahydrate formation compared to CaCl₂ of high purity. Neither was there any recognition by Carlsson et al., that impurities of NaCl and KCl in the composition could be beneficial in reducing tetrahydrate crystal formation in such phase change compositions.

Heat storage compositions are ideally packaged in individual encapsulating means for use in conjunction with solar heating systems. Exemplary of suitable known encapsulating means for the heat storage compositions herein described are water impervious films or foils of plastic/metal laminates. Closed cell plastic foams have also been suggested in which the PCM may be encapsulated within the cells of the foam structure as illustrated in, for example, U.S. Pat. No. 4,003,426. Other useful encapsulating means are concrete, metal or plastic containers, pipes, and the like.

SUMMARY OF THE INVENTION

The invention relates to reversible liquid/solid phase change compositions. More particularly, the invention resides in phase change compositions comprising a mixture of hydrated calcium chloride and potassium chloride in which the KCl modifies the semi-congruently melting behavior of CaCl₂.6H₂ O to the extent that the mixture approaches, and nearly reaches, congruent melting behavior. In a preferred application the phase change composition also includes the addition of NaCl and/or SrCl₂.6H₂ O to further modify the CaCl₂.6H₂ O/KCl mixture to thereby obtain a composition which is effectively a congruently melting composition. Optionally, the compositions of the invention also contain select nucleating additives to modify and suppress the supercooling properties of the liquid phase of the phase change compositions.

The present invention now recognizes that the addition of a predetermined amount of KCl to hydrated CaCl₂ substantially reduces the formation of crystal forms other than CaCl₂.6H₂ O thereby providing a CaCl₂ /KCl mixture in which the precipitation of crystal forms other than CaCl₂.6H₂ O on repeated melting and crystallization is substantially reduced. In addition, the hydrated CaCl₂ /KCl composition of the invention still provides a substantial cost advantage over other PCM's.

Although the hydrated CaCl₂ /KCl mixture of the invention surprisingly reduces the formation of crystal forms other than the hexahydrate form, it was found that it still retained the inherent characteristics of the supercooling properties of CaCl₂.6H₂ O. Accordingly, the present invention provides for the addition of select nucleating agents to the mixture thereby substantially improving the supercooling characteristics of the hydrated CaCl₂ /KCl system.

The avoidance of supercooling during the crystallization of hydrated CaCl₂, as by the addition of various nucleating agents, is known in the literature. Nucleating agents are substances upon which the phase change material crystal will grow with little or no supercooling. Accordingly, nucleating agents were not intended as additives to achieve congruently melting salt hydrates. Nucleators for hydrated CaCl₂ systems in particular are described in, for example, U.S.S.R. Inventorship Certificate No. 568,669, granted Mar. 3, 1975; Japanese Pat. No. 969,909, granted Aug. 31, 1979; and U.S. Pat. No. 4,189,394.

The present invention comprises a further improvement over the state of the art in that the nucleators of the invention are selective nucleators that will effectively reduce supercooling in the hydrated CaCl₂ /KCl system of the present invention.

Although the addition of less than about 8.0 weight percent KCl to CaCl₂.6H₂ O effectively reduces the tendency of the phase change composition to form, on freezing, the undesired phase CaCl₂.4H₂ O, even this amount is not quite sufficient to prevent completely the formation of CaCl₂.4H₂ O. Accordingly, in a preferred embodiment of the invention, the formation of CaCl₂.4H₂ O can be totally prevented from crystallizing if KCl is used in combination with NaCl and/or SrCl₂.6H₂ O. As hereinafter demonstrated, the addition of SrCl₂.6H₂ O to CaCl₂.6H₂ O to the full limit of its solubility does not prevent crystallization of CaCl₂.4H₂ O during freezing. However, as hereinafter further demonstrated, the addition of KCl plus NaCl; KCl plus SrCl₂.6H₂ O, or KCl plus NaCl and SrCl₂.6H₂ O modifies the phase equilibrium of CaCl₂.6H₂ O to fully supress the crystallization of CaCl₂.4H₂ O.

DETAILED DESCRIPTION OF THE INVENTION

The present invention represents a significant improvement towards the elusive goal of developing an inexpensive reversible liquid/solid phase change composition based on hydrated CaCl₂ in admixture with KCl in which the KCl is present in the hydrated CaCl₂ in an amount effective to reduce, during retrieval of the stored heat on freezing of the composition, formation of hydrated CaCl₂ crystalline phases other than CaCl₂.6H₂ O. The most basic composition of the invention comprises a mixture of from about 46 to about 52 weight percent CaCl₂ and from about 0.5 to about 8.0 weight percent KCl, with the balance being H₂ O (in an amount up to 100 weight percent). Preferably, the composition of the invention comprises a mixture of from about 47 to about 51 weight percent CaCl₂ and from about 2.3 to about 6.0 weight KCl with the balance being H₂ O (in an amount of up to 100 weight percent). Most preferably, the composition of the invention comprises a mixture of from about 48.0 to about 48.5 weight percent of CaCl₂ and from about 4.0 to about 4.7 weight percent KCl with the balance being H₂ O (in an amount of up to 100 weight percent).

Effective amounts of the selected nucleating agents for the hydrated CaCl₂ /KCl system of the invention are determined by testing a given composition over repetitive phase change cycles. While the following data illustrate that the nucleating agents produce marked benefits, such amounts should preferably not exceed 2.0 weight percent of the weight of the hydrated CaCl₂ /KCl mixture. Preferably, the nucleating agents are present in an amount of from about 0.005 to about 2.0 weight percent, based on the total weight of the composition. More preferably, the amount of the nucleating agents in the phase change composition is from about 0.01 to about 1.0 weight percent and most preferably, from about 0.10 to about 0.50 weight percent. It should be understood, however, that when reference is made to the addition of a given percentage of a nucleating agent, it is in addition to the amounts of ingredients already present in the phase change composition. Accordingly, the existing ingredient percentages are reduced proportionately to accomodate the addition of a nucleating agent(s).

Hydrated salt phase change materials exhibit three general types of phase/change behavior: congruent, semi-congruent and incongruent melting. The most desirable behavior is congruent melting which occurs when the solid phase change composition (ratio of salt to bound water) is the same as the liquid phase composition. In that case, the hydration/dehydration process appears identical to the melting and freezing process.

The term "effectively congruently melting mixture" herein used defines a mixture of ingredients, based on aqueous calcium chloride, for which, at the melting point, solid and liquid phases are in stable equilibrium: the solid phase containing no hydrated calcium chloride material other than the hexahydrate or solid solutions thereof; and the liquid phase containing, for every mole of calcium chloride, six moles of water, plus sufficient water to form the stable hydrate of any additive materials in solution.

Semi-congruent melting occurs when a phase change material has two or more hydrate forms with differing solid compositions and melting points. The material can be transformed into other hydrate forms before either complete melting or freezing occurs, resulting in a broadened melting point range. In addition, there is a temporary loss in thermal storage capacity. Calcium chloride hexahydrate is an example of a semi-congruently melting phase change material.

Incongruently melting phase change materials yield two distinct phases upon melting: a saturated solution and a precipitate of an insoluble anhydrous salt. If the precipitate settles out of the solution, the anhydrous salt will not hydrate completely upon cooling and some thermal storage capacity will be lost with each freezing/melting cycle. Incongruent melting, as observed with sodium sulfate decahydrate, for example, is a more serious problem because it can result in a continual loss of latent heat storage capacity.

The term "supercooling" refers to a discrepancy between the temperature at which freezing initiates and the melting temperature of a given liquid/solid phase change material when cooled and heated under quiescent conditions.

The term "additives" includes, in addition to nucleating agents such as have been specified hereinbelow, precursors of such additives which are non-detrimental to the function of the phase change materials of the invention. More particularly, the additives herein referred to are either anhydrous or hydrated compositions of inorganic salts or precursor materials which would form the salt upon addition to hydrated calcium chloride.

Impurities may be present in the phase change composition in minor amounts of less than 3.0 weight percent and provided that such impurities do not detrimentally affect the function of the basic hydrated CaCl₂ /KCl phase change compositions of the invention which may include the hereinafter specified nucleating agents and additives such as NaCl and/or SrCl₂.6H₂ O. Impurities may include, for example, alkaline earth or alkali metal halides such as CaBr₂ ; LiCl; MgCl₂, or other calcium salts such as CaCO₃ or CaSO₄.

Nucleating agents which have been found to be of particular benefit in the CaCl₂ /KCl mixture of the present invention are Ba(OH)₂, BaO, BaI₂, BaS₂ O₃, BaCO₃, BaCl₂, BaF₂, BaF₂.HF, Sr(OH)₂, SrO, SrCO₃, SrF₂, SrI₂, or mixtures thereof. Nucleators selected from BaCO₃ ; BaCl₂ ; BaO; Ba(OH)₂ ; BaI₂ ; BaSO₄ Sr(OH)₂, SrO, or mixtures thereof are preferred.

Compositions containing SrCl₂.6H₂ O as an additive or self-nucleating, unless heated to a temperature sufficient to dehydrate the SrCl₂.6H₂ O. Compositions not containing SrCl₂.6H₂ O as an additive should employ another nucleating additive, unless suitable mechanical means of nucleation are used.

The heat storage composition of the invention is preferably encapsulated in plastic containers such as the containers sold by Solar Inc., of Mead. Nebr. under the Trade Name SI Suntainer and described in Solar Engineering of April 1980, page 44. In accordance with the present invention, the reversible liquid-solid phase change composition is hermetically sealed in the encapsulating means to prevent evaporation of water from the composition. The composition comprises an admixture of hydrated CaCl₂ and KCl, wherein the KCl is added in an amount of less than 8.0 weight percent such that the weight ratio of KCl to CaCl₂ is from 1:50 to 1:5, the balance of the composition is H₂ O up to 100 weight percent. Preferably, the composition includes one or more nucleating agents in an amount of from about 0.005 to about 2.0 weight percent to reduce supercooling to 5° C. or less during retrieval of the stored heat by crystallization. The nucleating agents are selected from Ba(OH)₂, BaO, BaI₂, BaSO₄, BaS₂ O₃, BaCO₃, BaCl₂, BaF₂, BaF₂.HF, Sr(OH)₂, SrO, SrCO₃, SrF₂, SrI₂, or mixtures thereof. The composition may also include impurities in an amount of less than 3.0 weight percent.

The following examples illustrate the effectiveness of KCl for suppressing the formation of unwanted hydrates in the CaCl₂.6H₂ O/KCl phase change compositions of the invention.

EXAMPLE 1

883 Grams of a stirred liquid aqueous solution of 47.4 weight percent CaCl₂ was allowed to cool until a substantial quantity of CaCl₂.6H₂ O crystals was formed in the solution. 8.3 Grams of powdered KCl was then added and allowed to dissolve. When equilibrium was established (at 27.85° C.), an aliquot of the liquid phase was analyzed and found to contain 47.05 weight percent CaCl₂ and 0.95 weight percent KCl. Stepwise additions of a liquid aqueous solution of 55 weight percent CaCl₂ and powdered KCl were then made, allowing the system to come to equilibrium after each addition.

After 607.9 g of a 55 weight percent CaCl₂ solution and 4.4 powdered KCl had been added, and equilibrium was established at 29.02° C., an aliquot of the liquid phase showed 49.32 weight percent CaCl₂ and 0.96 weight percent KCl. Optical Microscopy showed only hexagonal CaCl₂.6H₂ O in the suspended solid phase.

After an additional 101.8 g of a 55 weight percent aqueous CaCl₂ and 1.2 g powdered KCl was added, the equilibrium temperature was 29.35° C., and the liquid phase contained 49.29 weight percent CaCl₂ and 0.95 weight percent KCl. Optical Microscopy showed only the triclinic crystals of CaCl₂.4H₂ O in the suspended solid phase.

Further additions of 55 weight percent aqueous CaCl₂ and powdered KCl showed that for a composition with a weight ratio of KCl:CaCl₂ being 1:50, and a mole ratio of H₂ O:CaCl₂ being 6:1, the equilibrium melting point of CaCl₂.4H₂ O is 32.1° C., and of CaCl₂.6H₂ O (extrapolated) is 29.2° C. Thus, a melted CaCl₂.6H₂ O sample with the above composition, if cooled, must cool through a temperature span of 2.9° C., during which crystallization of CaCl₂.4H₂ O is possible, before freezing of CaCl₂.6H₂ O can begin.

EXAMPLE 2

In the same manner as in Example 1, 932 g of 46.0 weight percent aqueous CaCl₂ and 20.69 g powdered KCl were brought into equilibrium with solid CaCl₂.6H₂ O at 25.85° C. The liquid phase contained 45.28 weight percent CaCl₂ and 2.46 weight percent KCl.

Stepwise addition of 55 weight percent aqueous CaCl₂ and powdered KCl resulted in a slurry of solid CaCl₂.6H₂ O in equilibrium at 28.13° C. with a solution containing 48.73 weight percent CaCl₂ and 2.64 weight percent KCl. The further addition of 55 weight percent CaCl₂ gave an equilibrium slurry (28.70° C.) of CaCl₂.4H₂ O in a solution with 48.98 weight percent CaCl₂ and 2.43 weight percent KCl.

Further additions of concentrated CaCl₂ solution and powdered KCl showed that for a composition with a weight ratio of KCl:CaCl₂ being 1:20, and a mole ratio of H₂ O:CaCl₂ being 6:1, the equilibrium melting point of CaCl₂.4H₂ O is 30.1° C., and of CaCl₂.6H₂ O (extrapolated) 28.1° C. A melted CaCl₂.6H₂ O sample of the above composition, in freezing, must cool, therefore, through a temperature span of 2.0° C., during which crystallization of CaCl₂.4H₂ O is possible, before freezing of CaCl₂.6H₂ O can begin.

EXAMPLE 3

In the same manner as in Examples 1 and 2, 889 g of 44.0 weight percent aqueous CaCl₂, saturated with KCl, was brought into equilibrium with solid CaCl₂.6H₂ O at 24.52° C. The liquid phase contained 44.16 weight percent CaCl₂ and 3.16 weight percent KCl.

Stepwise addition of an aqueous solution of 53.7 weight percent CaCl₂ l and 2.6 weight percent KCl, supplemented on occasion with powdered KCl, resulted in a slurry of solid CaCl₂.6H₂ O and KCl in equilibrium at 27.26° C. with a solution containing 48.21 weight percent CaCl₂ and 4.19 weight percent KCl. Further addition of the 53.7/2.6 weight percent CaCl₂ /KCl solution and powdered KCl yielded a slurry of solid CaCl₂.4H₂ O and KCl in equilibrium at 27.91° C. with a solution analyzing at 48.21 weight precent CaCl₂ and 4.48 weight percent KCl.

Further additions of the concentrated CaCl₂ /KCl solution and solid KCl showed that below a mole ratio of about 5.9:1, H₂ O:CaCl₂, KCl is completely soluble, if a weight ratio of KCl:CaCl₂ of 1:10 is maintained. At a 6:1 mole ratio of H₂ O:CaCl₂ and a weight ratio of KCl:CaCl₂ of 1:10, the equilibrium melting point of CaCl₂.4H₂ O is 28.0° C., and of CaCl₂.6H₂ O, 27.3° C. A liquefied sample of CaCl₂.6H₂ O of this composition, in freezing, must cool through a span of only 0.7° C., during which crystallization of CaCl₂.4H₂ O is possible, before freezing of CaCl₂.6H₂ O can occur.

EXAMPLE 4

In the same manner as in Examples 1, 2 and 3, a sample of aqueous CaCl₂ was studied, without the addition of any KCl. At a mole ratio of 6:1, H₂ O:CaCl₂, with no KCl present, the equilibrium melting point of CaCl₂.4H₂ O is 32.8° C., and of CaCl₂.6H₂ O is 29.6° C. Thus, a melted sample of CaCl₂.6H₂ O, in cooling, must pass through a span of 3.2° C., during which CaCl₂.4H₂ O crystallization is possible, before CaCl₂.6H₂ O freezing can begin.

                                      TABLE I                                      __________________________________________________________________________     Effect of Potassium Chloride on Calcium Chloride Hydrate Phases.sup.a          KCl/CaCl.sub.2                                                                        CaCl.sub.2.4H.sub.2 O                                                                  CaCl.sub.2.6H.sub.2 O                                                                  CaCl.sub.2.4H.sub.2 O                                                                  Max. Amt.                                       wt. ratio                                                                             m.p., °C.                                                                       m.p., °C.                                                                       Stable Span                                                                            CaCl.sub.2.4H.sub.2 O                           __________________________________________________________________________     0.sup.b                                                                               32.8    29.6    3.2° C.                                                                         9.45%                                           1:50   32.1    29.2    2.9° C.                                                                         7.58%                                           1:20   30.1    28.1    2.0° C.                                                                         5.88%                                           1:10   28.0    27.3    0.7° C.                                                                         1.86%                                           __________________________________________________________________________      .sup.a 6:1 mole ratio H.sub.2 O:CaCl.sub.2                                     .sup.b Not an example of the invention  Example 4                              m.p. -- melting point                                                    

Table I summarizes the findings of Examples 1-4 as to the equilibrium melting points of the tetrahydrate and hexahydrate of CaCl₂.

Table I also shows the maximum theoretical amount of CaCl₂.4H₂ O that could be formed in the freezing process. This was calculated using the "lever principle", from the peritectic compositions determined in the experiments outlined in Examples 1 to 4. It was surprisingly found that the addition of KCl not only reduces the temperature span within which CaCl₂.4H₂ O can be formed, but that it also reduces the maximum theoretical conversion of the CaCl₂.4H₂ O crystal form.

In practice, less than the theoretical amount of CaCl₂.4H₂ O is obtained. Both supercooling and slow crystal growth rate mitigate against tetrahydrate formation. For example, a composition of Example 3 containing 48.22 weight percent CaCl₂, 4.82 weight percent KCl, and 49.96 weight percent H₂ O would yield no CaCl₂.4H₂ O whatsoever, if the tetrahydrate supercooled as little as 0.7° C. and the hexahydrate were nucleated so as not to supercool. Further, in the experiments described in Examples 1, 2 and 3 above, it was noted that whenever CaCl₂.4H₂ O crystals were present, upon each addition of concentrated CaCl₂ solution, a much greater time period was required for temperature/composition equilibrium to be established than was the case in Example 4 which did not contain KCl. This illustrates the slow rate of CaCl₂.4H₂ O formation.

EXAMPLE 5

In order to determine the effects of nucleating agents on hydrated CaCl₂ /KCl heat storage materials, a number of 80 gram samples of CaCl₂.6H₂ O were prepared with varying levels of KCl added to the CaCl₂.6H₂ O. Selected nucleating agents were added to the hydrated CaCl₂ /KCl mixture to study their effects on supercooling.

For comparison, a control sample was prepared without the addition of a nucleating agent to the hydrated CaCl₂ /KCl storage material. The control sample contained 48.2 weight percent CaCl₂ ; 4.8 weight percent KCl and 47.0 weight percent water. In 4 of 10 freeze-thaw cycles, the control sample completely failed to freeze. In the remaining 6 cycles, an average supercooling temperature of greater than 14° C. was detected.

The results of the effect of KCl on barium salt nucleators for CaCl₂.6H₂ O are summarized in Table II.

                  TABLE II                                                         ______________________________________                                                     Supercooling, °C., Average                                  Nucleator              KCl:CaCl.sub.2                                                                            KCl:CaCl.sub.2                               Compound Wt. %   No KCl.sup.a                                                                             1:20     1:10                                       ______________________________________                                         None.sup.a                                                                              --      --        --       >14                                        Ba(OH).sub.2                                                                            2.00    --        --        2.5                                       "        1.00    --        0.3       4.9                                       "        0.50    0.0        0.25     9.5                                       "        0.10    0.16      --       --                                         "        0.05    0.55      --       --                                         "        0.01    1.92      --       --                                         "         0.005  4.45      7.5      --                                         BaO      1.00    --        0.7      >11                                        "        0.05    --         2.25    12.4                                       "        0.10    0.40      --       --                                         "        0.05    1.45      --       --                                         "        0.01    12.0      --       --                                         BaI.sub.2                                                                               1.00    --        1.6      >15                                        "        0.50    --        3.9      13.3                                       "        0.10    0         --       --                                         "        0.05    0.4       --       --                                         "        0.01    4.7       --       --                                         BaSO.sub.4                                                                              1.00    --        7.7      >16                                        "        0.50    --        8.0      17.7                                       "        0.10    0.26      --       --                                         "        0.05    7.4       --       --                                         BaCO.sub.3                                                                              1.00    --        2.3      >14                                        "        0.50    --        4.9      11.9                                       "        0.10    0         --       --                                         "        0.05    0         --       --                                         "        0.01    0.1       --       --                                         "         0.005  6.0       --       --                                         BaCl.sub.2                                                                              1.00    --        1.0      >15                                        "        0.50    0         11.0     12.3                                       "        0.10    1.03      --       --                                         "        0.01    1.38      --       --                                         ______________________________________                                          .sup.a Not an example of the invention.                                  

From Table II, it is evident that, in the absence of KCl, the amount of supercooling decreased as the level of the barium salt nucleator was increased. At a level of 0.5 weight percent, no supercooling was observed. In the case of BaCO₃ substantially no supercooling was observed at a level as low as 0.01 weight percent. For example containing KCl, the effectiveness of the nucleators diminished and a higher level of supercooling was observed as the KCl level was raised. Accordingly, it is evident from the data that higher levels of nucleators must be added in the presence of KCl to control supercooling. In general, depending on the barium salt selected as the nucleator, it is desirable to add at least 0.5 weight percent when the ratio of KCl:CaCl₂ is 1:20. When the ratio of KCl:CaCl₂ reached 1:10, it was found that the addition of a nucleator at that level produced no further benefits in reducing the level of supercooling. It would therefor be impractical to rely solely on any one of a select barium salt or barium salt mixture to control supercooling at a KCl:CaCl₂ ratio of 1:10 or greater.

The effects of strontium salt nucleators on the hydrated CaCl₂ /KCl composition of the invention are illustrated in Table III.

                  TABLE III                                                        ______________________________________                                         Effect of KCl on Strontium Salt Nucleators for CaCl.sub.2.6H.sub.2 O                      Supercooling, °C., Average                                   Nucleator              KCl:CaCl.sub.2                                                                            KCl:CaCl.sub.2                               Compound Wt. %   No KCl    1:20     1:10                                       ______________________________________                                         None     --      --        --       >14                                        SrCl.sub.2                                                                              2.00    --        --       0.1                                        "        1.00    0         --       --                                         "        0.50    6.3       --       --                                         "        0.10    >15       --       --                                         Sr(OH).sub.2                                                                            0.50    0         0        0                                          "        0.10    0.95      --       --                                         "        0.05    --        --       --                                         "        0.02    15.85     5.55     --                                         ______________________________________                                    

Table III illustrates that at the 2.0 weight percent level of SrCl₂, the amount of supercooling can be reduced nearly to zero, even in the presence of the maximum level of KCl. In the case of Sr(OH)₂, the addition of 0.5 weight percent surprisingly eliminated supercooling completely, even at the highest ratio of KCl:CaCl₂ of 1:10.

Table IV shows the combined effect of barium salts and strontium salts on supercooling of hydrated CaCl₂ and KCl heat storage compound.

                  TABLE IV                                                         ______________________________________                                         Combined Effect of Binary Nucleators on CaCl.sub.2.6H.sub.2 O                             Supercooling Average, °C.                                    Nucleator    No KCl.sup.(a)                                                                           KCl:CaCl.sub.2 :SrCl.sub.2                              Compound Wt. %   No SrCl.sub.2                                                                            5:100:2.sup.(c)                                                                       5:100:1                                                                              10:100:1                               ______________________________________                                         Ba(OH).sub.2                                                                            1.00    --        --     --    0                                      "        0.50    0         0      0     4.2                                    "        0.10    0.18      0.30   --    --                                     "        0.05    0.69      0.78   --    --                                     "        0.01    1.92      1.72   --    --                                     BaO      0.50    0         --     0     3.8                                    "        0.10    0.32      0.58   --    --                                     BaCO.sub.3                                                                              0.50    .sup.(b)  --     0     3.3                                    BaSO.sub.4                                                                              0.50    .sup.(b)  --       1.1 6.3                                    BaCl.sub.2                                                                              0.50    0         0        0.2 0.4                                    "        0.10    0.40      0.98   --    --                                     "        0.01    --        1.38   --    --                                     BaI.sub.2                                                                               0.50    .sup.(b)  --     0     7.0                                    None     --      11.9      7.7    --    --                                     ______________________________________                                          .sup.(a) Not an example of the invention                                       .sup.(b) Assumed to be 0, since the lower level of nucleator yielded           0° supercooling                                                         .sup.(c) NaCl:CaCl.sub.2 (1:100).                                        

From Table IV, it will be noted that the presence of the binary barium/strontium salt nucleator had the unexpected property of suppressing supercooling even in the presence of a high level of KCl. Very little difference was observed between the results for nucleated pure CaCl₂.6H₂ O and nucleated CaCl₂.6H₂ O containing KCl and SrCl₂. Since KCl is known to reduce the effectiveness of barium salts, the lack of supercooling can be attributed to the action of SrCl₂ in reversing the effect of KCl on nucleators.

Table V illustrates the most effective combination of barium salt or strontium salt nucleators, or mixtures thereof, and weight percentages on supercooling of KCl/CaCl₂ mixtures at ratios of 1:20 and 1:10.

                  TABLE V                                                          ______________________________________                                         Nucleator(s)     Supercooling, °C., Average                                       Weight     KCl:CaCl.sub.2 =                                                                          KCl:CaCl.sub.2 =                               Compound  Percent    1:20       1:10                                           ______________________________________                                         Ba(OH).sub.2                                                                             0.5         0.25                                                     BaO       1.0        0.7                                                       BaO       0.5        0.0                                                       SrCl.sub.2                                                                               0.5                                                                  BaI.sub.2 1.0        1.6                                                       BaI.sub.2 0.5        0.0                                                       SrCI.sub.2                                                                               0.5                                                                  BaSO.sub.4                                                                               0.5        1.1                                                       SrCl.sub.2                                                                     BaCO.sub.3                                                                               0.5        0.0                                                       SrCl.sub.2                                                                               0.5                                                                  BaCl.sub.2                                                                               1.0        1.0                                                       BaCl.sub.2                                                                               0.5        0.2        0.4                                            SrCl.sub.2                                                                               0.5                                                                  Sr(OH).sub.2                                                                             0.5                   0.0                                            ______________________________________                                    

From Table V, it will be seen that the addition of BaI₂ suppresses supercooling to 1.6° C. and although higher than the remaining listed nucleators, still represents a highly effective material. Surprisinly, the mixture of BaI₂ with SrCl₂ suggests a synergistic effect since 0° C. supercooling was obtained.

EXAMPLE 6

The following Example involves saturation of CaCl₂.6H₂ O with NaCl. It shows that there is but a slight effect on the potential formation of tetrahydrate from this additive alone.

In the same procedural manner as in Example 2, 1230 g of a 47.47 weight percent aqueous CaCl₂ solution and 12.4 g NaCl powder were mixed and brought to equilibrium with solid CaCl₂.6H₂ O at 28.13° C. The liquid phase contained 47.28 weight percent CaCl₂, 52.43 weight percent H₂ O, and 0.29 weight percent NaCl by analysis.

Stepwise addition of an aqueous solution of 55.15 weight percent CaCl₂ resulted in a slurry of solid CaCl₂.6H₂ O and NaCl in metastable equilibrium at 29.40° C. with a solution of 49.92 weight percent CaCl₂, 49.84 weight percent H₂ O, and 0.24 weight percent NaCl, by analysis. Further addition of the 55.15 weight percent solution of CaCl₂ yielded a slurry of solid CaCl₂.4H₂ O and NaCl in equilibrium at 31.48° C. with a solution of 50.14 weight percent CaCl₂, 49.60 weight percent H₂ O, and 0.26 weight percent NaCl.

Further additions of the concentrated CaCl₂ solution showed that at a 6:1 mole ratio of H₂ O:CaCl₂, saturated with NaCl, the equilibrium melting point of CaCl₂.4H₂ O is 32.8° C., and the metastable equilibrium melting point of CaCl₂.6H₂ O is 29.4° C. A liquefied sample of CaCl₂.6H₂ O of this composition, in freezing, must cool through a span of 3.4° C., during which the crystallization of CaCl₂.4H₂ O is possible, before freezing of CaCl₂.6H₂ O can begin. Thus, saturation with NaCl causes a slight increase in the tendency to form tetrahydrate.

EXAMPLE 7

This Example involves saturation of CaCl₂.4H₂ O with both KCl and NaCl, and it is an example of the present invention. It shows that although NaCl addition causes a slight increase in CaCl₂.4H₂ O formation, and KCl addition markedly reduces, but does not eliminate CaCl₂.4H₂ O formation, the combination of KCl and NaCl additives totally suppresses tetrahydrate formation.

In the same procedural manner as in Example 2, 898 g of a mixture of 46 weight percent CaCl₂, 47 weight percent H₂ O, 6 weight percent KCl, and 1 weight percent NaCl was heated, cooled, and brought to equilibrium at 26.02° C. CaCl₂.6H₂ O, KCl, and NaCl crystals were in equilibrium with a solution of 45.65 weight percent CaCl₂, 49.5 weight percent H₂ O, 3.9 weight percent KCl, and 9.95 weight percent NaCl.

Stepwise addition of an aqueous solution of 53.0 weight percent CaCl₂, 5.5 weight prcent KCl, and 41.5 weight percent H₂ O resulted in a slurry of CaCl₂.6H₂ O in equilibrium at 26.90° C. with a solution containing 47.90 weight percent CaCl₂, 46.6 weight percent H₂ O, 4.7 weight percent KCl, and 0.83 weight percent NaCl. Further addition of the 53.0/5.5 weight percent CaCl₂ /KCl solution and 5 g powdered NaCl yielded a slurry of CaCl₂.4H₂ O in equilibrium at 27.82° C. with a solution analyzing 48.15 weight percent CaCl₂, 46.2 weight percent H₂ O, 4.8 weight percent KCl, and 0.86 weight percent NaCl.

Further addition of the concentrated CaCl₂ /KCl solution, powdered NaCl, and powdered KCl demonstrated that at a 6:1 mole ratio of H₂ O:CaCl₂, saturated with KCl and NaCl, the equilibrium melting point of CaCl₂.6H₂ O is 27.0° C., and the metastable equilibrium melting point of CaCl₂.4H₂ O is about 26.7° C. A liquefied sample of CaCl₂.6H₂ O of this composition, in freezing, will crystallize to solid CaCl₂.6H₂ O, without the possibility of CaCl₂.4H₂ O formation.

EXAMPLE 8

This Example is not an example of the invention and demonstrates that CaCl₂.6H₂ O saturated with SrCl₂.6H₂ O decreases but cannot eliminate the potential for tetrahydrate formation.

In the same procedural manner as in Example 2, 748 g of a mixture of 45 weight percent CaCl₂, 53 weight percent H₂ O, and 2 weight percent SrCl₂ was heated, cooled, and brought to equilibrium at 25.91° C. Crystals of SrCl₂.2H₂ O and CaCl₂.6H₂ O/SrCl₂.6H₂ O solid solution were in equilibrium with liquid analyzing 44.18 weight percent CaCl₂, 55.37 weight percent H₂ O, and 0.55 weight percent SrCl₂.

Stepwise addition of a solution of 54.3 weight percent CaCl₂, 43.7 weight percent H₂ O, and 2.0 weight percent SrCl₂ resulted in a slurry of CaCl₂.6H₂ O/SrCl₂.6H₂ O solid solution and SrCl₂.2H₂ O crystals in metastable equilibrium at 30.28° C. with liquid analyzing 50.08 weight percent CaCl₂, 49.22 weight percent H₂ O, and 0.70 weight percent SrCl₂.

Further stepwise addition of concentrated CaCl₂ /SrCl₂ solution, followed by cooling, resulted in a slurry of CaCl₂.4H₂ O and SrCl₂.2H₂ O in equilibrium at 30.50° C. with liquid analyzing 49.70 weight percent CaCl₂, 49.66 weight percent H₂ O, and 0.50 weight percent SrCl₂.

Further experiments of this type showed that at a mole ratio of 6:1 of H₂ O:(CaCl₂ +SrCl₂), the equilibrium melting point of CaCl₂.4H₂ O is 32.1° C., and the metastable equilibrium melting point of a CaCl₂.6H₂ O (solid solution) is 30.4° C. A liquefied sample of CaCl₂.6H₂ O of this composition, in freezing must cool through a span of 1.7° C., during which CaCl₂.4H₂ O can freeze, before crystallization of CaCl₂.6H₂ O can begin. Thus, SrCl₂.6H₂ O additive reduces the tendency of the tetrahydrate to form (roughly by half), but does not prevent it entirely.

EXAMPLE 9

The following example involves the saturation of CaCl₂.6H₂ O with both KCl and SrCl₂.6H₂ O, and is an example of the present invention. This Example demonstrates that the addition of both KCl and SrCl₂.6H₂ O to CaCl₂.6H₂ O totally reduces tetrahydrate formation.

In the same procedural manner as in Example 2, 757 g of a mixture of 43.4 weight percent CaCl₂, 4.3 weight percent KCl, 1.9 weight percent SrCl₂, and 50.3 weight percent H₂ O was heated, cooled, and brought to equilibrium at 27.18° C. Crystals of KCl, SrCl₂.2H₂ O and CaCl₂.6H₂ O/SrCl₂.6H₂ O were in equilibrium in a liquid which was analyzed to contain 44.21 weight percent CaCl₂, 3.9 weght percent KCl, 1.15 weight percent SrCl₂, and 50.7 weight percent H₂ O.

Stepwise addition of a solution containing 51.7 weight percent CaCl₂, 5.2 weight percent KCl, 1.0 weight percent SrCl₂ and 42.2 weight percent H₂ O led to a slurry of CaCl₂.6H₂ O/SrCl₂.6H₂ O solid solution, SrCl₂.2H₂ O and KCl crystals in equilibrium at 27.68° C. The liquid was analyzed to contain 47.57 weight percent CaCl₂, 47.02 weight percent H₂ O, 4.65 weight percent KCl, and 0.76 weight percent SrCl₂.

Further stepwise additions of concentrated CaCl₂ /KCl/SrCl₂ solution, followed by cooling, led to a slurry of CaCl₂.4H₂ O, SrCl₂.2H₂ O, and KCl in equilibrium at 27.54° C. The liquid was analyzed to contain 48.03 weight percent CaCl₂, 46.45 weight percent H₂ O, 4.9 weight percent KCl, and 0.62 weight percent SrCl₂.

Further experiments of this type showed that at a mole ratio of 6:1 of H₂ O:(CaCl₂ +SrCl₂), the equilibrium melting point of CaCl₂.6H₂ O (solid solution) is 27.8° C., and the metastable equilibrium melting point of CaCl₂.4H₂ O is about 27° C. A liquefied sample of CaCl₂.6H₂ O of this composition, in freezing, will crystallize to crystalline CaCl₂.6H₂ O/SrCl₂.6H₂ O solid solution without the possibility of tetrahydrate formation.

EXAMPLE 10

This Example illustrates saturation of CaCl₂.6H₂ O with KCl, SrCl₂.6H₂ O, and NaCl, and is an example of the present invention. Since tetrahydrate crystallization in CaCl₂.6H₂ O can be suppressed by adding KCl+SrCl₂ or KCl+NaCl, but not SrCl₂ +NaCl, this Example surprisingly demonstrates that addition of all three salts would be beneficial in completely suppressing the tetrahydrate formation.

In the same procedural manner as in Example 2, 761 g of a mixture of 43.16 weight percent CaCl₂, 49.52 weight percent H₂ O, 4.37 weight percent KCl, 1.96 weight percent SrCl₂, and 0.99 weight percent NaCl was heated, cooled, and brought to equilibrium at 26.62° C. Crystals of CaCl₂.6H₂ O/SrCl₂.6H₂ O, SrCl₂.2H₂ O, KCl, and NaCl were in equilibrium. The liquid was analyzed to contain 43.98 weight percent CaCl₂, 50.78 weight percent H₂ O, 3.90 weight percent KCl, 1.02 weight percent SrCl₂, and 0.32 weight percent NaCl.

Stepwise addition of a solution containing 51.8 weight percent CaCl₂, 41.5 weight percent H₂ O, 5.2 weight percent KCl, 1.0 weight percent SrCl₂, and 0.5 weight percent NaCl led to a slurry of CaCl₂.6H₂ O/SrCl₂.6H₂ O, SrCl₂.2H₂ O, KCl, and NaCl in equilibrium at 27.56° C. The liquid was analyzed to contain 47.52 weight percent CaCl₂, 46.62 weight percent H₂ O, 4.75 weight percent KCl, 0.83 weight percent SrCl₂, and 0.28 weight percent NaCl.

Further stepwise addition of the concentrated CaCl₂ /KCl/SrCl₂ /NaCl solution led to a slurry of CaCl₂.4H₂ O, SrCl₂.2H₂ O, KCl, and NaCl in equilibrium. The liquid was analyzed to contain 48.48 weight percent CaCl₂, 45.68 weight percent H₂ O, 5.33 weight percent KCl, 0.84 weight percent SrCl₂, and 0.26 weight percent NaCl, at a temperature of 27.77° C.

At a mole ratio of 6:1 of H₂ O:(CaCl₂ +SrCl₂), the equilibrium melting point of CaCl₂.6H₂ O (solid solution) is 27.5° C., and the metastable equilibrium melting point of CaCl₂.4H₂ O is about 26° C. A liquefied sample of CaCl₂.6H₂ O of this composition, in freezing, will crystallize to solid CaCl₂.6H₂ O/SrCl₂.6H₂ O solid solution without the possibility of tetrahydrate formation.

Table VI summarizes the results of previous Examples 3 and 4 and new Examples 6 through 10 and the effects of additives on calcium hydrate phase at a 6:1 mole ratio, H₂ O:(CaCl₂ +SrCl₂), saturated with the indicated additive(s).

                  TABLE VI                                                         ______________________________________                                         Effect of Additives on Calcium Chloride Hydrate Phases                                     CaCl.sub.2.4H.sub.2 O                                                                     CaCl.sub.2.6H.sub.2 O                                                                     CaCl.sub.2.4H.sub.2 O                        Additive    m.p., °C.                                                                          m.p., °C.                                                                          Stable Span                                  ______________________________________                                         4   none        32.8       29.6     3.2° C.                             3   KCl         28.0       27.3     0.7° C.                             6   NaCl        32.8       29.4     3.4° C.                             7   KCl + NaCl  26.7       27.0     none                                       8   SrCl.sub.2  32.1       30.4     1.7° C.                             9   KCl + SrCl.sub.2                                                                           27         27.8     none                                       10  KCl +       26         27.5     none                                           SrCl.sub.2 + NaCl                                                          ______________________________________                                    

EXAMPLE 11

As a further demonstration of the elimination of CaCl₂.4H₂ O formation by the present invention, and as a comparison with other means of tetrahydrate supression, a number of CaCl₂.6H₂ O compositions with additives were prepared. These samples were melted, and cooled to a temperature barely above the melting point of CaCl₂.6H₂ O, as determined by the experiments in the preceding Examples. After a period of equilibration, a seed crystal of CaCl₂.4H₂ O was added, and its effect observed. If formation of tetrahydrate were possible for these compositions, the seed crystal would persist or grow. Otherwise, it would dissolve. The results of these experiments are illustrated in Table VII in which the CaCl₂.4H₂ O seeding was conducted at a 6:1 mole ratio of H₂ O:(CaCl₂ +SrCl₂), saturated with the indicated additive(s). These results confirm the present invention.

                  TABLE VII                                                        ______________________________________                                         Additive    Temp. °C.                                                                            Result of Seeding                                     ______________________________________                                         KCl         27.5         CaCl.sub.2.4H.sub.2 O growth                          SrCl.sub.2  31.0         CaCl.sub.2.4H.sub.2 O growth                          KCl + SrCl.sub.2.sup.a                                                                     27.8         Seed dissolved                                        KCl + NaCl.sup.a                                                                           26.9         Seed dissolved                                        ______________________________________                                          .sup.a Examples of the present invention.                                

EXAMPLE 12

The experiment of Example 11 was repeated, using a sample prepared by following the procedures of Swedish Pat. No. 410,004.

148 Grams of CaCl₂.6H₂ O was prepared, containing 50.66 weight percent CaCl₂ and 49.43 weight percent H₂ O. Then 2.97 g (2 percent by weight) SrCl₂.6H₂ O was added and dissolved at 60° C. After equilibration at 32° C. for 23 hours, the solution was filtered, and the test of Example 11 was performed at 30.6° C. Seeds of CaCl₂.4H₂ O grew in the solution. After two days the remaining liquid was found to contain 49.30 percent CaCl₂, 50.31 percent H₂ O, and 0.39 percent SrCl₂ by weight, a mole ratio of 6.252:1 of H₂ O:(CaCl₂ +SrCl₂).

This experiment demonstrates that the composition is not a congruently melting composition.

SUMMARY OF EXAMPLES 6 TO 12

As discussed hereinbefore, the addition of SrCl₂.6H₂ O to the full limit of its solubility in CaCl₂.6H₂ O does not prevent the the crystallization of CaCl₂.4H₂ O during freezing of such composition. When SrCl₂.6H₂ O is added, the melting point of CaCl₂.6H₂ O increases (due to formation of an isomorphous solid solution), and the melting point of CaCl₂.4H₂ O decreases. Again, the freezing process is like that for pure CaCl₂.6H₂ O, except that less tetrahydrate can form, and that solid solution freezes in place of the hexahydrate. At the solubility limit of about 1.0 weight percent SrCl₂.6H₂ O, freezing of the tetrahydrate begins at 32.2° C., and the peritectic reaction isotherm is at 30.6° C. It is estimated that if about 2.1 weight percent SrCl₂.6H₂ O were dissolved, no tetrahydrate could form. However, the limit of solubility is about 1.0 weight percent. Accordingly, the effect of SrCl₂.6H₂ O on the freezing of CaCl₂.6H₂ O is as predicted by Carlsson, et al. but because of the solubility limit of SrCl₂.6H₂ O their predicted results cannot be achieved.

In accordance with the present invention, it has surprisingly been found that about 0.8 weight percent NaCl can be added to CaCl₂.6H₂ O saturated with KCl. This reduces the melting point of CaCl₂.6H₂ O, but reduces faster the melting point of CaCl₂.4H₂ O. The combined effect of the two additives is to prevent any tetrahydrate formation. The mixture melts at 27.0° C. The CaCl₂.6H₂ O/CaCl₂.4H₂ O eutectic point is at 27.0° C. at the CaCl₂.5.98H₂ O stoichiometry.

In accordance with the invention, it has also been surprisingly found that when KCl is added to CaCl₂.6H₂ O saturated with SrCl₂.6H₂ O, the melting point decreases. That of the tetrahydrate is affected more than that of the hexahydrate. The following data indicate that about 3.25 weight percent KCl will effectively prevent tetrahydrate formation. The solubility limit is about 4.75 weight percent, so that there is no problem in obtaining the necessary concentration. The mixture, saturated with KCl and SrCl₂.6H₂ O, melts at 27.8° C. The hexahydrate/tetrahydrate eutectic point is at 27.6° C. and CaCl₂.5.91H₂ O stoichiometry.

Further, in accordance with the invention, it has been found that about 4.8 weight percent KCl, plus 0.3 weight percent NaCl, plus 0.8 weight percent SrCl₂.6H₂ O are soluble in CaCl₂.6H₂ O and that tetrahydrate crystallization is completely prevented. The mixture melts at 27.5° C. There is a eutectic point at 27.4° C. at the stoichiometry (CaCl₂ +SrCl₂).5.82H₂ O.

In Example 4, it was demonstrated that for CaCl₂.6H₂ O without additives, considerable tetrahydrate formation is possible during freezing.

In Example 3, it was demonstrated that by saturating CaCl₂.6H₂ O with KCl, the potential for tetrahydrate formation is reduced markedly and that KCl surprisingly modified the semi-congruently melting behavior of CaCl₂.6H₂ O to the extent that the mixture nearly reaches congruent melting behavior. 

What is claimed is:
 1. A reversible liquid/solid phase change composition comprising an admixture of hydrated CaCl₂ and KCl, wherein KCl is added to the hydrated CaCl₂ in an amount sufficient to modify the semi-congruent melting behavior of CaCl₂.6H₂ O to the extent that the mixture approaches the congruent melting behavior of a congruently melting mixture and to reduce, during retrieval of the stored heat by crystallization of the mixture, the formation of crystalline CaCl₂ hydrate phases other than CaCl₂.6H₂ O.
 2. The composition of claim 1, comprising from about 46 to about 52 weight percent CaCl₂ having KCl added to the CaCl₂ such that the KCl is present in an amount of from about 0.5 to about 8 weight percent, with the balance being H₂ O (up to 100 weight percent).
 3. The composition of claim 1 or 2, comprising from about 48.0 to about 48.5 weight percent CaCl₂, having KCl added to the CaCl₂ such that the KCl is present in an amount of from about 4.0 to about 4.7 weight percent with the balance being H₂ O (up to 100 weight percent).
 4. The composition of claim 1, wherein the weight ratio of KCl to CaCl₂ in the composition is from 1:50 to 1:5 and wherein the mole ratio of the H₂ O to CaCl₂ in the composition is about 6:1.
 5. The composition of claim 4, wherein the weight ratio of KCl to CaCl₂ in the composition is from 1:15 to 1:10.
 6. The composition of claim 1, including an amount of NaCl added to the hydrated CaCl₂ sufficient to obtain an effectively congruently melting mixture.
 7. The composition of claim 6, wherein the weight ratio of NaCl to CaCl₂ in the composition is from 1:40 to 1:70.
 8. The composition of claim 7, wherein the weight ratio of NaCl to CaCl₂ in the composition is from 1:45 to 1:65.
 9. The composition of claim 1 or 6, including an amount of SrCl₂.6H₂ O added to the hydrated CaCl₂ sufficient to obtain an effectively congruently melting mixture.
 10. The composition of claim 9, wherein the mole ratio of H₂ O to CaCl₂ plus SrCl₂ in the composition is about 6:1.
 11. The composition of claim 9, wherein the weight ratio of SrCl₂.6H₂ O to CaCl₂ in the composition is from 1:50 to 1:110.
 12. The composition of claim 11, wherein the weight ratio of SrCl₂.6H₂ O to CaCl₂ in the composition is from 1:75 to 1:100.
 13. The composition of claim 9, wherein the weight ratio of SrCl₂.6H₂ O plus NaCl to CaCl₂ in the composition is from 1:40 to 1:70.
 14. The composition of claim 1, including the addition of one or more nucleating agents in said composition in an amount of from about 0.005 to about 2.0 weight percent to reduce supercooling to 5° C. or less during retrieval of the stored heat by crystallization.
 15. The composition of claim 14, wherein the nucleating agent is present in an amount of from about 0.10 to about 1.0 weight percent.
 16. The composition of claim 14 or 15, wherein the nucleating agent is selected from Ba(OH)₂, BaO, BaI₂, BaSO₄, BaS₂ O₃, BaCO₃, BaCl₂, BaF₂, BaF₂.HF, Sr(OH)₂, SrO, SrCO₃, SrF₂, SrI₂, or mixtures thereof.
 17. The composition of claim 16, wherein the nucleating agent is selected from Ba(OH)₂ ; BaO; BaI₂ ; BaSO₄ ; BaCO₃ ; BaCl₂ ; Sr(OH)₂ ; or mixtures thereof.
 18. The composition of claim 1, wherein said composition includes impurities in an amount of less thaan 3.0 weight percent.
 19. In a reversible liquid/solid phase change composition comprising a mixture of CaCl₂.6H₂ O/KCl/NaCl/SrCl₂.6H₂ O and impurities in an amount of less than 3 weight percent of the total weight of the composition, the improvement comprising the addition of KCl and NaCl in amounts sufficient to obtain an effectively congruently melting mixture.
 20. The composition of claim 19, the improvement comprising from about 46 to about 52 weight percent CaCl₂, having KCl added to the CaCl₂ such that the KCl is present in an amount of from about 0.5 to about 8 weight percent with the balance being H₂ O (up to 100 weight percent).
 21. The composition of claim 19, the improvement comprising a weight ratio of KCl and CaCl₂ in the composition of from 1:50 to 1:5, a weight ratio of NaCl to CaCl₂ in the composition of from 1:40 to 1:70, and a mole ratio of H₂ O to CaCl₂ in the composition of about 6:1.
 22. The composition of claim 19, the improvement comprising the addition of SrCl₂.6H₂ O to the composition in an amount sufficient to obtain a weight ratio of SrCl₂.6H₂ O to CaCl₂ of from 1:50 to 1:110.
 23. The composition of claim 19, the improvement comprising the addition of SrCl₂.6H₂ O plus NaCl to CaCl₂ in an amount sufficient to obtain a weight ratio of from 1:40 to 1:70.
 24. The composition of claim 19, the improvement comprising the addition of one or more nucleating agents to said composition in an amount of from about 0.005 to about 2.0 weight percent to reduce supercooling to 5° C. or less during retrieval of the stored heat by crystallization.
 25. The composition of claim 24, the improvement comprising a nucleating agent selected from Ba(OH)₂, BaO, BaI₂, BaSO₄, BaS₂ O₃, BaCO₃, BaCl₂, BaF₂, BaF₂.HF, Sr(OH)₂, SrO, SrCO₃, SrF₂, SrI₂, or mixtures thereof.
 26. The composition of claim 19, comprising the improvement of adding a sufficient amount of KCl to CaCl₂ to obtain a mixture of from about 48.0 to about 48.5 weight percent CaCl₂ and from about 4.0 to about 4.7 weight percent KCl, and water in an amount to obtain a mole ratio of H₂ O to CaCl₂ of about 6:1.
 27. The composition of claim 19, comprising the improvement of adding SrCl₂.6H₂ O in an amount such that the weight ratio of SrCl₂.6H₂ O plus NaCl to CaCl₂ in the composition is from 1:40 to 1:70 and adding one or more nucleating agents in said composition in an amount of from about 0.005 to about 2.0 weight percent to reduce supercooling to 5° C. or less during retrieval of the stored heat by crystallization, said nucleating agents being selected from Ba(OH)₂ ; BaO; BaI₂ ; BaSO₄ ; BaCO₃ ; BaCl₂ ; Sr(OH)₂ ; or mixtures thereof. PG,61
 28. A heat storage device comprising an encapsulating means having a reversible liquid-solid phase change composition hermetically sealed in said encapsulating means to prevent evaporation of water from the composition, said composition comprising an admixture of hydrated CaCl₂ and KCl, wherein KCl is added to the hydrated CaCl₂ in an amount of less than 8.0 weight percent such that the weight ratio of KCl to CaCl₂ is from 1:50 to 1:5, the balance of said composition being H₂ O up to 100 weight percent.
 29. The heat storage device of claim 28, including an amount of NaCl added to the hydrated CaCl₂ sufficient to obtain an effectively congruently melting mixture.
 30. The heat storage device of claim 29, wherein the weight ratio of NaCl to CaCl₂ is from 1:40 to 1:70.
 31. The heat storage device of claim 28 or 29, including an amount of SrCl₂ added to the hydrated CaCl₂ sufficient to obtain an effectively congruently melting mixture.
 32. The heat storage device of claim 31, wherein the weight ratio of SrCl₂.6H₂ O to CaCl₂ in the composition is from 1:50 to 1:110.
 33. The heat storage device of claim 31, wherein the weight ratio of SrCl₂.6H₂ O plus NaCl to CaCl₂ in the composition is from 1:40 to 1:70.
 34. The heat storage device of claim 28, including one or more nucleating agents added to the hydrated CaCl₂ in an amount of from about 0.005 to about 2.0 weight percent to reduce supercooling to 5° C. or less during retrieval of the stored heat by crystallization, said nucleating agents being selected from Ba(OH)₂, BaO, BaI₂, BaSO₄, BaS₂ O₃, BaCO₃, BaCl₂, BaF₂, BaF₂.HF, Sr(OH)₂, SrO, SrCO₃, SrF₂, SrI₂, or mixtures thereof.
 35. The heat storage device of claim 28, wherein said composition includes impurities in an amount of less than 3.0 weight percent.
 36. A method of storing heat, comprising the steps of preparing a reversible liquid-solid phase change composition by admixing hydrated CaCl₂ and KCl, comprising the steps of adding less than 8 weight percent KCl to the hydrated CaCl₂ such that the weight ratio of KCl to CaCl₂ in the composition is from 1:50 to 1:5 and the balance being water up to 100 percent, introducing the composition into an encapsulating means for use as a heat storage device, and hermetically sealing the encapsulating means to prevent evaporation of water from the composition.
 37. The method of claim 36, including the step of adding to the composition in an amount sufficient to obtain an effectively congruently melting mixture, wherein the weight ratio of NaCl to CaCl₂ is from 1:40 to 1:70.
 38. The method of claim 36, including the step of adding to the composition in an amount sufficient to obtain an effectively congruently melting mixture, wherein the weight ratio of SrCl₂.6H₂ O to CaCl₂ in the composition is from 1:50 to 1:110.
 39. The method of claim 36, including the step of adding NaCl and SrCl₂ to the composition in an amount wherein the weight ratio of SrCl₂.6H₂ O plus NaCl to CaCl₂ in the composition is from 1:40 to 1:70.
 40. The method of claim 36, wherein the said composition includes impurities in an amount of less than 3.0 weight percent.
 41. A heat storage material which comprises a composition of CaCl₂.6H₂ O modified for preventing a crystallization of CaCl₂.4H₂ O and less than about 8.0 weight percent KCl alone or in admixture with a compound selected from the group consisting of NaCl, SrCl₂.6H₂ O and a mixture thereof.
 42. The heat storage material of claim 41, wherein said composition modified for preventing a crystallization of CaCl₂.4H₂ O comprises CaCl₂ hydrate having a water content at a molar ratio of about 4.74 to about 7.17, based on CaCl₂.
 43. The heat storage material of claim 42, wherein the CaCl₂ hydrate has a water content at a molar ratio of from about 5.93 to about 6.16, based on CaCl₂. 